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What Is an Index
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In the previous chapter, you learned about tables, which are, in essence, repositories that hold data and information about data what it looks like and where it is held. However, a table definition is not a great deal of use in getting to the data quickly. For this, some sort of cross-reference
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CHAPTER 6 CREATING INDEXES AND DATABASE DIAGRAMMING
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facility is required, where for certain columns of information within a table it should be possible to get to the whole record of information quickly. If you think of this book, for example, as a table, the cross-reference you would use to find information quickly is the index at the back of the book. You look in the book index for a piece of information, or key. When you find the listing for that information in the index, you ll find it s associated with a page number, or a pointer, which directs you to where you can find the data you re looking for. This is where an index within your SQL Server database comes in. You define an index in SQL Server so that it can locate the rows it requires to satisfy database queries faster. If an index does not exist to help find the necessary rows, SQL Server has no other option but to look at every row in a table to see if it contains the information required by the query. This is called a table scan, which by its very nature adds considerable overhead to data-retrieval operations.
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Note There will be times when a table scan is the preferred option over an index. For example, if SQL Server needs to process a reasonable proportion of rows within a table, sometimes estimated to be around 10 percent or more of the data, then it may find that using a table scan is better than using an index. This is all to say that a table scan isn t wholly a bad thing.
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When searching a table using the index, SQL Server does not go through all the data stored in the table; rather, it focuses on a much smaller subset of that data, as it will be looking at the columns defined within the index, which is faster. Once the record is found in the index, a pointer states where the data for that row can be found in the relevant table. There are different types of indexes you can build onto a table. An index can be created on one column, called a simple index, or on more than one column, called a compound index. The circumstances of the column or columns you select and the data that will be held within these columns determine which type of index you use.
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Types of Indexes
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Although SQL Server has three types of indexes clustered, nonclustered, and primary and secondary XML indexes we will concentrate only on clustered and nonclustered in this book, as XML and XML indexes are quite an advanced topic. The index type refers to the way the index and the physical rows of data are stored internally by SQL Server. The differences between the index types are important to understand, so we ll delve into them in the sections that follow.
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Clustered
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A clustered index defines the physical order of the data in the table. If you have more than one column defined in a clustered index, the data will be stored in sequential order according to columns: the first column, then the next column, and so on. Only one clustered index can be defined per table. It would be impossible to store the data in two different physical orders. Going back to our earlier book analogy, if you examine a telephone book, you ll see that the data is presented in alphabetical order with surnames appearing first, then first names, and then
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